1. Field of the Invention
The present invention relates to an offset measuring mechanism and an offset measuring method in a bonding apparatus and more particularly to an offset measuring mechanism and offset measuring method for measuring the offset between the position detection camera and bonding tool in a bonding apparatus.
2. Prior Art
In a wire bonding apparatus in which, for instance, a semiconductor device and a circuit board are connected by a fine wire, the bonding positions on the semiconductor device are detected by a position detection camera, and bonding work is performed by moving the bonding tool to these positions. In this case, if the bonding tool is disposed so that the axial center of the bonding tool coincides with the optical axis of the position detection camera, the visual field of position detection is blocked; ordinarily, therefore, the position detection camera and the bonding tool are disposed with a specified distance in between. This specified distance between the optical axis of the position detection camera and the axial center of the bonding tool is called an “offset”, and is an important quantity which serves as a reference for positioning in the bonding work. However, this offset varies from instant to instant as a result of changes over time in the amount of radiant heat from the high-temperature bonding stage and the amount of heat generated in the optical system, etc., or in the amount of wear in the moving parts used for bonding, etc.
Japanese Patent No. 2982000 (Japanese Patent Application Laid-Open (Kokai) No. 2000-21923) discloses a method that determines the accurate amount of offset by a reference member, which is fixed with respect to the bonding apparatus, and an offset correction camera, which observes this reference member. More specifically, the optical axis of the position detection camera is first moved to a point above the reference member, and the relative positions of the reference member and position detection camera are measured. Next, the bonding tool is moved to a point above the reference member in accordance with a specified amount of offset, and the relative positions of the reference member and the bonding tool are measured by the offset correction camera. Then, the accurate amount of offset is determined by correcting the amount of offset on the bases of these measurement results.
On the other hand, Japanese Patent Application Laid-Open (Kokai) No. 2001-203234 discloses a method that makes it possible to measure the relative positions of a reference member and the bonding tool by means of a position detection camera, without using an offset correction camera. More specifically, instead of an offset correction camera, an optical system is constructed by optical elements such as laser diodes, half-mirrors and prisms, etc., and image light of the reference member and bonding tool is conducted to the position detection camera, and then the relative positions of the reference member and boding tool are measured by the position detection camera.
In the offset correction methods of the above-described prior arts, a reference member is provided, and the position of the position detection camera and the position of the bonding tool are respectively measured using the reference member as a reference, and the amount of offset is determined based upon these measurement results. Accordingly, the reference member is fixed in a reference position outside the position detection camera or offset correcting camera; and thus contamination of the reference member occurs. Furthermore, in order to obtain accurate measurements of the relative positions of the reference member and bonding tool, images of both the tip end of the bonding tool and the reference member must be clearly measured. Accordingly, the bonding tool must be caused to approach the reference member as closely as possible; and there is a danger that the bonding tool may collide with re reference member during this approach operation, thus damaging the reference member.
As seen from the above, in the prior art offset measuring methods, since the reference member used as a reference for position measurement is exposed to the outside, contamination and damage, etc. would occur in the reference member, and thus it is difficult to perform periodic offset measurements in a stable fashion.
Furthermore, since caution is required in the handling of the reference member, which is exposed to the outside, the degree of freedom regarding the disposition of the respective constituent elements of the bonding apparatus is limited.